Milan Sljivic
Full Professor
Faculty of Mechanical Engineering,
University of Banja Luka,
Bosnia and Herzegovina
Ana Pavlovic
Assistant Professor
Department of Industrial Engineering
Alma Mater Studiorum University of
Bologna,
Italy
Mico Stanojevic
Republic Agency for SME
Banja Luka,
Bosnia and Herzegovina
Cristiano Fragassa
Assistant Professor
Department of Industrial engineering,
Alma Mater Studiorum University of
Bologna,
Italy
1.
Combining Additive Manufacturing and
Vacuum Casting for an Efficient
Manufacturing of Safety Glasses
This paper presents the possibility of integration of modern techniques of
additive manufacturing and vacuum casting in the rapid manufacturing of
complex parts. The prototype of safety glasses was directly developed from
a CAD model, which is used as a main model for making moulds in a
vacuum casting process. The development stages, which include additive
production of glasses and the process of vacuum casting with the definition
of material components, preheating temperature, the forming of silicone
mould, the casting process, hardening and other parameters are presented
in this paper. It is proved that by using this technique it is possible to
produce complex functional parts quickly, with high precision, accuracy
and surface quality, while significantly reducing costs of development and
production compared to other similar technologies.
Keywords: Additive manufacturing, Vacuum casting, Rapid prototyping,
safety glasses
INTRODUCTION
The more time spent on the product development, the
more opportunities for profit are lost. It is this
philosophy that drives many industries during the
development of new products [1]. Recognizing the need
to create more prototypes of the same part, efforts have
been made to develop a cost-effective system of rapid
prototyping of replicas of parts. This system would
allow users to check design, quality and other tests
before the start of mass production and presentation of
products on the market. Thus, this method of rapid
production combines two manufacturing processes,
rapid prototyping and rapid tooling.
a.
Requirements and need for an integrated system
Rapid prototyping technology, as well as many other
technologies, has a range of limitations [2]. Limits in
usage of rapid prototyping techniques are mainly related
to high costs of using these systems. These costs are
generally related to the purchase, amortization and
maintenance of RP systems, the cost of materials and
the cost of involvement of qualified personnel. In
addition, one of the main drawbacks of RP technique is
a limited number of materials that are used, since each
of the RP techniques generally uses a specific type of
material. In the actual case, a 3D printer, based on the
principle of extrusion materials (Fused Deposition
Molding - FDM).
This material is often not the same or even similar
to the material that a finished product needs to be made
of. Also, mechanical and physical properties of the
Received: May 2016, Accepted: June 2016
Correspondence to: Prof. Ana Pavlovic
Department of Industrial Engineering, Alma Mater
Studiorum University of Bologna, Italy
E-mail: [email protected]
doi:10.5937/fmet1604393S
© Faculty of Mechanical Engineering, Belgrade. All rights reserved
material generally do not coincide with the
characteristics of the material a final product should be
made of [3-5]. For functional and final testing, of the
utmost importance is that the physical and mechanical
properties of the material used for making prototypes
are the same or at least close to the characteristics of
materials that will be used to produce the final product,
which is a requirement that the materials used by the RP
systems often do not meet. At the stage of development
and design of the product, when performing basic
analysis and opinions, it is often required to use a
certain number of identical prototypes for testing. At
this stage, therefore, it is necessary to produce a small
series of identical prototypes, which, using the 3D
printer, can be very slow, costly and inefficient. This is
another shortcoming of RP techniques, and it also
reveals the need to develop a system that will allow the
production of small series of identical prototypes. The
alternative is an integrated system which will use the 3D
printer to produce a prototype of the product, which
would be used in the next step as a master model for
tools that would produce a series of prototypes, i.e.
products made of different materials, with
characteristics close to the finished product. The
integrated system of rapid prototyping and rapid tooling,
in this case the technology of vacuum casting and 3D
printers based on the principle of FDM (Fig.1.), shall
satisfy the needs and requirements of engineers [6-9].
Engineers perform the assessment of the functionality
and characteristics of the prototype that needs to satisfy
the following requirements:
• The prototype should look like the final product in
terms of dimensional accuracy, surface quality and
colour.
• It should be made of materials with the same or
similar characteristics as the final product to be
tested in real working conditions. Also, there
should be no voids in the internal structure.
FME Transactions (2016) 44, 393-397 393
•
The prototypes should be made in sufficient
quantities to facilitate and accelerate the process of
design optimization and should be mutually equal.
The layout of model of safety glasses designed in
SolidWorks software package is shown in Figure 2 and
parameter adjustment within the CatalystEX, where the
selection of the orientation of models, processing of
layers and layout of models on a base on which printing
on a "Dimension Elite" 3D printer will be performed, is
shown in Figure 3.
Figure 1: An integrated system
2.
THE ROLE OF 3D PRINTING IN AN INTEGRATED
PROCESS
The RP technique, which was used in a specific case for
making master models of safety glasses, is a 3D printer
based on FDM principle (Stratasys - Dimension Elite),
which plays a key role in the integrated process. Of
great importance is the accuracy that is achieved in this
3D printer, during which it is necessary to pay special
attention to the positioning and orientation of the master
model on platform [6]. The procedure that has been
necessary to obtain a master model or prototype of
safety glasses is as follows:
• Product design in some of the CAD software
packages,
• Conversion of CAD models in STL format that is
recognized by a 3D printer,
• Transfer of STL files to the computer that controls
the three-dimensional printer,
• Processing of STL files within the CatalystEX
program in which all the parameters are set and
adjusted according to the required model,
• Creating a three-dimensional model using additive
technology [10] and
• Further processing of created prototypes.
Figure 2: A model of safety glasses in the Solid Works
software package
394 ▪ VOL. 44, No 4, 2016
Figure 3: Orientation and processing of models in Catalyst-EX
3.
THE ROLE OF VACUUM CASTING IN AN
INTEGRATED PROCESS
The technology of vacuum casting has become a widely
accepted rapid tooling technique. Vacuum casting is a
newer version of investment casting, differing in the
process of making moulds [11-13]. The most important
feature of vacuum casting is a significant reduction in
the time needed to produce a small series of parts as
well as reducing the cost compared to traditional
methods. The example of making silicone molds and
casting replicas of safety glasses according to the master
model developed in a 3D printer, on the equipment for
vacuum casting (MK-Technology, Germany) is used to
present the process of vacuum casting [14,15].
The process of vacuum casting consists of the
following steps:
1. Preparation of negatives of safety glasses produced
FME Transactions
2.
3.
4.
on a 3D printer for the casting process.
Setting the parting tape on the negative of safety
glasses that will facilitate the separation of silicone
mould.
Bonding plastic gates on the negative of safety
glasses which has a role to form an inlet channel in
the silicone mould and to facilitate the positioning
and fixation of the negative in the silicone casting
frame, according to Figure 4a.
Calculation of the necessary quantity of silicone
mixture to form a silicone mould to be used for
detail the elements of safety glasses. The silicone
mixture is poured into the frame with fixed
negatives of safety glasses and then the frame with
the negative submerged in silicone is placed in a
vacuum chamber in order to remove the residual air
bubbles from silicon, according to Figure 4b.
Table 1: Physical characteristics of components used for
casting wings of safety glasses
PHYSICAL PROPERTIES PX 223 HT
PART A
PART B MIXING
Composition
ISOCYANATE POLYOL
Mixing ratio by
100
80
weight
Aspect
liquid
liquid
liquid
Colour
colourless
black
black
Viscosity at
BROOKFIELD
1.100
300
850
77°F (25°C)
LVT
(mPa s)
Density of parts
1.17
1.12
before mixing at ISO 1675:1975
25°C
Density of cured
ISO 2781:1988
1.14
mixing at 23°C
Pot life at
6-7
Table 2: Mechanical characteristics of components used
for casting wings of safety glasses
MECHANICAL PROPERTIES AT 23°C PX 223 HT
Flexural modules of elasticity
Flexural strength
Tensile strength
Elongation
Charpy impact resistance
Izod Impact - Notched
Izod Impact - Unnotched
Hardness - at 73°F (23°C)
- at 248°F (120°C)
a)
b)
Figure 4: (a) Casting silicone into the frame with the
negative, and (b) removing the residual air from the silicon
in a vacuum chamber
5.
6.
Positioning of the silicone mould in a vacuum
chamber (Figure 5a). After solidification of
silicone, silicone mould is cut to parting line,
during which we relieve the negative and get a
silicone mould for casting a replica of a given
negative (Figure 5b).
The moulding halves are then combined and the
next step is to calculate necessary quantities of resin
for moulding elements of safety glasses. The
amount of resin is commonly determined by
weighing the individual master model, which is
increased by 20-30%, taking into account the loss
of material in vessels and inlet channels. In this
case, to cast the wings of safety glasses,
components made by Axson Technologies were
used, thus by mixing them in the casting process
parts with physical and mechanical characteristics
according to Table 1 and Table 2 are obtained.
MPa
MPa
MPa
ISO 527-2:1993
%
ISO 179/2D:1994 kJ/m2
ASTM D256-05 kJ/m2
ASTM D256-05 kJ/m2
Shore
ISO 868:2003
D1
2.300
80
60
11
>601
6
161
80
>65
ISO 178:2001
For casting protective glass of glasses are also used
components of manufacturers Axson Technologies
which mixed in casting process allows for obtaining
parts with characteristics such as transparency, high
precision, good UV protection and convenience for
finishing, according to Table 3 and Table 4.
Table 3: Physical characteristics of components used for
casting the protective glass of glasses
PHYSICAL PROPERTIES PX 223 HT
Composition
Mixing ratio by
weight
Aspect
Colour
Viscosity at
BROOKFIELD
LVT
25°C (mPa s)
Density at 25°C ISO 1675:1975
Density of
cured mixing at ISO 2781:1988
23°C
Pot life at 25°C
on 155 g (min)
ISOCYANATE
PX 521HT A
100
POLYOL
PX 521 MIXING
HT B
80
liquid
Transparent
liquid
Bluish
liquid
transparent
200
1.100
500
1.07
1.05
-
-
-
1.06
20
Table 4: Mechanical characteristics of components used
for casting the protective glass of glasses
MECHANICAL PROPERTIES AT 23°C PX 223 HT
a)
b)
Figure 5: a) Positioning a silicone mould in a vacuum
chamber and b) Separated silicone mould
FME Transactions
Flexural modules of elasticity
Flexural strength
Tensile modulus of elasticity
Tensile strength
Elongation at break point in
tenstion
Charpy impact resistance
Izod Impact - Notched
Izod Impact - Unnotched
Hardness
ISO 178:2001
ISO 527-2:1993
MPa
MPa
MPa
MPa
2.100
105
2.700
75
%
9
2
ISO 179/1eU:1994 kJ/m
ASTM D256-05
kJ/m2
ASTM D256-05
kJ/m2
ISO 868:1965 Shore D1
27
6
17
87
VOL. 44, No 4, 2016 ▪ 395
7.
After a certain quantity of the material needed for
moulding and the proportion of the individual
components of the material in a total amount is
determined, then vacuum casting process follows.
The casting process takes place in a vacuum
chamber under conditions that are recommended
for corresponding elements and components of the
material, according to Figure 6.
Figure 6: The casting process in a vacuum chamber MKTechnology
8.
After solidification of the moulded material in a
vacuum chamber, mould halves are separated and,
if necessary, post-processing of the moulded item
follows.
Different safety glasses were made in the integrated
process of additive manufacturing technology and
vacuum casting in the Laboratory for Plasticity and
Processing Systems at the Faculty of Mechanical
Engineering in Banja Luka and at the Department of
Industrial Engineering at University of Bologna, where
result of this study are shown in Figure 7.
elements in these areas. The design of safety glasses
yielded specific characteristics such as good surface
quality, satisfactory physical appearance, transparency,
rigidity, elasticity, strength, hardness, temperature
resistance, UV stability, colour and so on. In terms of
cost and time of production, it has been proven that
these parameters were significantly reduced compared
to the comparative technology. As demonstrated in
several studies (as in [16]), the additive manufacturing
can represent the right option for a large gamma of
industrial products. Furthermore, additional savings
could be realized using an integrated approach for the
full cost optimization of the process as a whole, instead
of moving toward single steps of reduction for additive
manufacturing and, separately, vacuum casting.
Finally, the benefits provided by these new
techniques toward a modern manufacturing has to be
also considered in term of eco-sustainability, evident
when compared with conventional methods ([17]).
ACKNOWLEDGEMENT
This investigation on application of advanced
technologies for sustainable additive manufacturing
methods and technologies has been realized as part of
Tempus WIMB project implementation, a project cofunded by the European Union and IPA CBC AdriaHub
project [18].
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[3] Wohlers Report 2014: 3D Printing and Additive
Manufacturing State of the Industry Annual
Worldwide Progress Report, Wohlesrs Associates,
INC. Colorado 80525 USA.
Figure 7: Safety glasses made in the integrated process of
additive manufacturing technology and vacuum casting
4.
CONCLUSIONS
The development of safety glasses through the
integration process of additive manufacturing and
vacuum casting shows an example of a very successful
application of modern technology in the rapid
development of prototypes and production small series
of identical prototypes. Prototypes can be made from
the original material that can be very broad spectrum in
terms of physical, technical and mechanical properties.
Considering that this integrated technology has
applications in many industrial fields such as
automotive, aerospace, pharmaceutical, medical, food
industry and others, an example of creating safety
glasses confirms its application to very complex
396 ▪ VOL. 44, No 4, 2016
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КОМБИНАЦИЈА АДИТИВНЕ ПРОИЗВОДЊЕ
И ВАКУУМ ЛИВЕЊА ЗА ЕФИКАСНУ
ПРОИЗВОДЊУ ЗАШТИТНИХ НАОЧАРА
М. Шљивић, А. Павловић, М. Станојевић,
К. Фрагаса
Овај рад приказује могућност интеграције
савремених техника адитивне производње и вакуум
ливења у брзој производњи сложених делова.
Прототип сигурносних наочара је директно развијен
из ЦАД модела, који се користи као главни модел за
израду калупа у процесу вакуум ливења. У овом
раду приказана је развојна фаза, која укључује
адитивну производњу наочара и процес вакуум
ливења са дефинисањем материјала компонента,
предгревање
температуром,
формирање
силиконских калупа, процес ливења, каљење и друге
параметре. Доказано је да је могуће помоћу ове
технике произвести сложене функционалне делове
брзо, са високом прецизношћу, тачношћу и
квалитетом површине, док су трошкови развоја и
производње у односу на друге сличне технологије
значајно смањени.
VOL. 44, No 4, 2016 ▪ 397